According to a few people Safari is messing up font sizing because it uses 72ppi for display instead of the more common 96ppi.

As Matthew Haughey states:
“My biggest problem with Safari is that it seems to default to a 72dpi display instead of every other modern browser's 96dpi.”

Well they're wrong, and here's a screen shot to prove it:

And here's the file I used in this testing. This file declares no fonts and no font sizes and thus would fall victim to the “problem” mentioned above.

I use Verdana 13 as my base font and size and as you can see there is no difference in the displaying of this.

So what are these people see when they set no font size and use Safari to browse their sites? Well it is quite simple. Safari defaults to a size 14 font, Lucida Grande 14 to be exact. Most modern browsers default to a size 16 font, Chimera does Lucida Grande 16 and MacIE uses Times 16, I believe.

When the designer declares a font face with no size the browser uses the user's selected font size, in Safari's case it's a step smaller than the typical default size. That's where the mistaken belief that the 72ppi resolution is somehow responsible for this.

Do I know if Safari uses 72ppi?

I have no idea, but I do know the reason it displays font sizes differently is because people aren't changing its preferences to match their current browser preferences.